The present invention relates to instrumentation apparatus and methods utilizing a light-modulated photoconductor as a dielectric, lens, mirror, or similar structure. Such light-modulated photoconductor exhibits a variable permittivity, which variable permittivity allows the light-modulated photoconductor to be used for many and varied applications. As a dielectric, for example, the light-modulated photoconductor may be used between conductive plates or surfaces to provide a light-modulated capacitor, or "photocapacitor". Such photocapacitor may be incorporated into instrumentation used to, e.g., measure the work function of surfaces during processing, or as a non-contact voltmeter. As a lens or mirror, the light-modulated photoconductor provides a means for optically modulating the index of refraction of a lens or mirror assembly. Such lens or mirror assembly may be incorporated into a beam steering device, e.g., a solid state infra-red (IR) beam steering device.
Many modern processes require precise surface properties. In order to prepare well characterized surfaces, a versatile instrument is needed that can probe surface properties during device processing under a variety of environmental conditions. One technique known in the art for determining surface properties is to measure the work function.
The work function of a material is defined as the energy that must be supplied to remove an electron at the Fermi level through the surface to a point in space outside the body far enough away that the image potential is negligible. There are two contributions to the work function of a material: the Fermi energy and the surface potential barrier.
Because the work function depends on the surface potential barrier, it is a sensitive measure of surface physical and chemical properties. For example, the work function varies with the actual crystal plane exposed at the surface by an amount typically of several tenths of an electron volt (eV) out of an overall typical magnitude of two to six eV. Work function variations due to the presence of adsorbed surface species or changes in composition at the surface are even larger. These surface chemical changes produce variations in work function of up to several eV. For these reasons, the work function is an extremely useful probe of surface properties.
Further, many applications require devices which are sensitive to the surface properties. These devices depend either directly or indirectly on the work function for their operation. All thermionic and photoemissive devices, for example, clearly depend on the work function for their operation. Further, the work function indirectly enters the operation of many integrated circuit devices which depend on current flow across internal junctions. In addition, superconducting quantum interference devices (SQUIDS) depend indirectly on the work function of the two materials comprising the device. Hence, the work function becomes an extremely useful parameter to measure and monitor.
There is also an ever increasing need to monitor the high T.sub.c superconductor surface electrical properties during processing of tunnel junction structures and high frequency cavity structures. Further, there is often a need to monitor surfaces of semiconductor devices during processing (in-situ wafer processing), not just before and after processing. In addition, there is a great need to monitor surfaces during surface cleaning operations. These and other specific needs emphasize the overall need for a monitoring device that does not contact the surfaces being monitored, so that, e.g., high temperatures, wafer processing, or surface cleaning operations may proceed relatively unimpeded.
In general, the instrumentation devices and methods known in the art for measuring the work function include: (1) photoemission; (2) thermionic emission; (3) field emission; and (4) contact potential difference (CPD). The first three of these techniques disadvantageously require a high vacuum for their operation. The CPD method can be carried out by either: (a) a diode technique, which requires a high vacuum, or (b) a variable capacitance technique. Because creating a high vacuum requires a significant amount of auxiliary equipment, which auxiliary equipment is usually expensive and burdensome to acquire, maintain, and operate, the variable capacitance CPD method is usually preferred.
In the variable capacitance method, the CPD is usually measured by the technique of Kelvin and Zisman, as described, e.g., in Holzl and F.K. Schulte, in Solid Surface Phvsics, edited by G. Hohler, Vol. 85 of "Springer Tracts in Modern Physics", pp. 1-150 (Springer, Berlin 1979). This method depends upon the fact that when two dissimilar materials are brought into electrical contact, a charge flows until the chemical potential of the conduction electrons are equal. The resulting electrostatic voltage between the two conductors, the CPD, is measured by varying the capacitance between them. This is explained more fully below in Appendix A.
Instrumentation apparatus employing the variable capacitance CPD method must provide a capacitor having a capacitance value that can be easily modulated at a desired frequency. Prior art techniques for modulating a capacitor have typically employed a vibrating capacitor wherein the capacitor plate spacing is mechanically varied. Unfortunately, mechanical factors limit the frequency of vibration of such capacitors to typically a few hundred Hz. In addition, with mechanical vibration at any frequency, stray capacitance to surrounding components produces a time varying signal that is indistinguishable from the signal of interest. Higher frequencies of modulation would lower the noise and provide a greater resolution of the measurement. Hence, what is needed to efficiently and accurately carry out the CPD method of measuring the work function is a capacitor that can be modulated at high frequencies, ideally by non-mechanical means.
Some existing instruments provide probes using piezoelectric elements to vary the plate spacing. See, e.g., Baumgartner et al., "Micro Kelvin Probe for Local Work-Function Measurements," Rev. Sci. Instrum. 59, 802 (1988); Besocke et al., "Piezoeletric Driven Kelvin Probe for Contact Potential Difference Studies," Rev. Sci. Instrum. 47, 840 (1976). In theory, such elements could oscillate at Mhz frequencies, but in practice are typically limited to frequencies just below 1 Khz. This is because at the higher frequencies the vibration amplitudes become very small and the driving voltages become very large (approaching a kilovolt). Such large driving voltages pose problems of interference with the CPD measurement. Moreover, as previously mentioned, any mechanical vibration, regardless of the frequency, tends to create spurious signals through stray capacitance to surrounding components.
Other existing commercial instruments that utilize a variable capacitor operate in the low audio range. These instruments use vibrating reed or tuning forks in sensor heads. The heads require careful shielding for millivolt sensitivity and are therefore bulky. What is needed for low level (millivolt sensitivity) measurements is a compact non-mechanical high frequency method for capacitance variation. However, all known methods of varying capacitance rely on mechanical methods, e.g., vibrating, rotating, pendulum, driven oscillating wire, etc. There are no non-mechanical methods of varying the capacitance known to applicant, except one, and that relies on a radioactive source to generate an ion current, and only provides resolution to about 1 volt. Thus, there is a clear need for a non-mechanical method of varying a capacitor.
It is noted that in U.S. Pat. No. 4,812,756 (Curtis et al.), a contactless technique for measuring the voltage changes between a top surface oxide layer and a bulk semiconductor wafer during processing is disclosed. However, the technique disclosed utilizes a conventional Kelvin probe apparatus employing a vibrating pick-up plate to vary the capacitance. For some measurements, Curtis et al. teach that the vibrating pick-up plate may be transparent in order to allow photons to be applied to the semiconductor wafer. Curtis et al. further emphasize the many and varied measurements that should be made on a semiconductor wafer in order to determine its suitability for further device processing. However, no teachings are provided by Curtis et al. relating to how the vibrating pick-up plate could be replaced with a non-vibrating plate, which is really what is needed if a non-mechanical variable capacitor structure is to be provided.
The present invention advantageously addresses the above and other needs by providing a capacitor structure wherein the capacitance value between a surface being monitored and a reference electrode may be varied by non-mechanical means.